The present exemplary embodiment relates to a pump system, and more particularly to a system that includes a high speed centrifugal pump and a variable displacement pump. It finds particular application in conjunction with a fuel metering system, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Generally, a high speed centrifugal pump is desired in a pump system because of the benefits related to power consumption. Further, the centrifugal pump is generally lightweight, is deemed to have an extended life, has limited pressure ripple, and is generally conducive for a wide array of downstream uses. When trying to apply high-speed centrifugal pump technology in connection with fuel pumps associated with aircraft engine applications, for example, certain conditions need attention. Engine start is one area because the high speed centrifugal pump does not provide the required elevated fuel pressures for start-up until the engine speed has increased to a certain level. Therefore, there is a need to start the engine with a relatively low drive speed. However, the pump system can become excessively large for the balance of pump operating conditions, i.e., idle, cruise, climb, etc. if engine start-up were the only parameter concern. A large pump, in turn, creates a series of system issues revolving around the thermal impacts on the system. Likewise, there are size constraints and weight issues that must be addressed in designing an effective fuel pump system.
Therefore, a variable displacement pump is often incorporated into the pump system to address the low pressure rise from the centrifugal pump at low engine speed. The variable displacement pump provides the additional required pressure and flow at engine start. In addition to using the variable displacement pumping stage to provide engine start, i.e. satisfy engine start requirements, the variable displacement pump likewise delivers pressures beyond the maximum pressure delivered by the centrifugal stage when needed for other uses.
In order to provide accurate metered flow, and reduce the pressure to a level required downstream, a centrifugal based pump and metering system works based on pressure generation from the centrifugal pump and a throttling style metering system. However, for the variable displacement pump to work in the throttling style metering system, the variable displacement pump must be pressure compensated. This would allow the centrifugal pump at low speed to combine with the additional pressure and flow offered by the variable displacement pump to meet start-up requirements. Further, the variable displacement pump can then be advantageously used to provide a pressure increase above the discharge of the centrifugal pump output once the centrifugal pump has reached a maximum speed and maximum pressure output and there is a need for additional output.
Accordingly, a need exists for providing additional system capability (and associated method) to a pump unit that includes a centrifugal pump and throttling style metering system in a manner that is economical, effective, compact, simple, automatic, pressure compensated, and adaptable to one or more metering loops.